There has been a short time where the potions didn't do anything. I noticed the exact same thing with one of my batches, but I think it was only a glitch because other onces I had did start to work again. The idea is basically to give you a positive effect (Resistance IV) combined with dozens of negative effects (mostly cosmetic) to rule it out.

Weakness and Resistance do not cancel each other out btw. Resistance reduces all incoming damage, except void. Weakness reduces all damage you do with melee attacks. You can see an overview of all Minecraft status effects on their official wiki here: http://minecraft.gamepedia.com/Status_effect